The present invention relates to a power transmission system, particularly to a hydromechanical transmission for self-propelled machines, such as industrial and farming vehicles and similar machines.
It is known that various hydromechanical transmissions have already been devised for vehicles and machines assigned to special tasks, such as backhoes, loaders, excavators, container carriers, lifting cranes, farming machines, and the like; all these transmissions have the purpose of distributing the usable power between a mechanical branch with gears and a hydrostatic branch (hydrostatic motor and hydrostatic pump), thus achieving the advantages offered by hydrostatic power transmission, which in practice allows great speed control, together with the advantages provided by purely mechanical power transmission, which in practice offers high efficiency and reliability.
However, these hydromechanical transmissions are not always able to provide a range of reduction ratios such as required by the above-mentioned machines; therefore, in order to obviate this drawback, one or more reduction units with a high number of speed ranges are associated at the output of the hydromechanical transmission; this obviously entails a considerable effect on costs, complexity, and overall bulk.
As an alternative to these hydromechanical transmissions, a type of CVT transmission (i.e., a continuously variable transmission known to technicians as I.sup.2) has already been proposed which substantially uses four clutches arranged in pairs on opposite sides of a CVT unit, for example a hydrostatic unit constituted by a hydrostatic motor and by a corresponding hydrostatic pump, both having a variable and adjustable volume or displacement, so as to be able to initially connect the pump to the power source (generally a Diesel engine) and the hydrostatic motor to the output shaft (transmission shaft) and then, when the hydrostatic motor reaches its maximum speed, to connect, by means of the second pair of said clutches, said hydrostatic motor to the power source (Diesel), making it work like a hydrostatic pump, and simultaneously so as to be able to connect the pump to the output shaft, reducing its displacement until its maximum rotation rate is reached.
This type of hydrostatic transmission therefore allows to shift from the minimum speed to the maximum speed by appropriately adjusting the displacement of the pump and of the hydrostatic motor; in practice, however, its total "range" is not always sufficient for certain industrial vehicles, which must travel on roads and comply with set speed limits.